I'm meeting my stepsis and I don't know what to feel.

You're never "too old to feel bad". You feel what you feel. What a silly assumption.

So, feel what you feel, and recognize that your stepsister (interesting how your mother kept everything from you - letters, checks, etc. - for I am assuming over a decade and a half) is innocent in all of this.

Go. Be kind. Say hi. Expect fully well that it will be 100% awkward. Meet in a public place and for a limited time, say, for coffee and a danish. Nothing more. There's no need for big revelations or tearful hugs. Treat her like you would treat meeting any near-stranger, like a fellow student or a new coworker (I have no idea how old you are so I can't tell exactly what the analogue would be).

Note: you never need to meet again if you don't want to. But a one-time meeting for 15 minutes in a public place won't hurt anything. And it just might be the validation your stepsister needs. Do it as a kindness and consider it your good deed for the month.